This invention relates generally to small aperture antennas and in particular to devices for reducing the backlobe radiation from such antennas.
The recent conflict in Southeast Asia has demonstrated that aircraft are particularly susceptible to attack from the rear either by surface-to-air missiles, or air-to-air missiles, or by hostile aircraft. Obviously then, a tail warning radar system, which will provide the pilot with warning of attacks from the rear in sufficient time to initiate countermeasures deployment and employ either engaging or evasive tactics, is desirable. The antenna for such a radar system should provide a broad radiation pattern directed to the rear of the aircraft and sould be capable of providing both a sum and difference pattern so that the position, either to the left or right of the aircraft centerline, of the attacker can be determined. In addition, the radar system should be designed to maximize missile detections at all closing velocities, including closing velocities less than aircraft speed. As at low altitudes ground clutter returns can enter the radar system with the same Doppler frequencies as targets of interest, an antenna with low backlobes is required to reduce the sensitivity of the radar system to clutter returns. In fact, backlobe levels on the order of 40 dB below the beam peak are required if targets with closing velocities less than the aircraft velocity are to be detected.
While there exists many prior art methods of controlling the backlobe radiation of antennas, none are particularly well suited for airborne applications where, as here, the allowable packaging volume is severely limited. Thus, antennas utilizing multimode apertures (as, for example, a scalar horn antenna) can provide low backlobe radiation, but they are much too large. Traveling wave antennas such as dielectric rod antennas, log periodic antennas, or slot array antennas can provide the 40 dB backlobe levels required, but their main lobe beamwidths are much too narrow and their length becomes excessive for aircraft tail mounting applications.
Other prior art methods of reducing undesired backlobe radiation include the use of chokes around the aperture of the antenna. While chokes may be effective in reducing the backlobe radiation for the antenna sum pattern, their effect on the antenna difference pattern backlobes is not as pronounced.
A circular waveguide radiator, having an effective aperture of approximately 0.82.lambda., where .lambda. is the wavelength of the centerband frequency of the antenna, was chosen as the optimum antenna design for the given application. While such an antenna has the broad beamwidth required, and provides a rugged assembly suitable for aircraft applications, its backlobe radiation is only approximately 20 dB below that of the beam peak. Obviously then, a means for reducing both the sum and difference pattern backlobes from such an antenna is required.